This morning I tried turning on the van but it wouldnt start. It happen last week but I had a mechanic take a look at it but he said nothing was wrong with it. When he got it to his shop it started up with out any problems. When I brought it home it was working fine but now its having the same problem. After trying to turn it on I noticed that it smells like gas. The smells comes from the rear drivers side.

Check spark,fuel pressure.
Look closely at the coil wire and the cap and rotor on those.

As my '97 fuel pump got weak, it would cold start on starting fluid, but then ran fine, and would start when warm.
A quick spray of carb cleaner or so would tell you if it's a spark problem, or a fuel pressure problem.
To start, a GM service rep told me " minimum 60" psi. I had 59, but that wasn't good enough.

If you smell fuel: Check fuel pressure while the pump is running, as Drew300 mentioned, 60psi MINIMUM. After the pump shuts down, (approximately 3 seconds) watch for leakdown. Pressure should not drop below 55 psi. If it does, you have a leak somewhere.